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Confirmation of manometer outcome

user-1129453 | Posted in General Questions on

I am looking to get a ball park estimate of the cfm of a range hood exhaust in a restaurant. I saw that to do a bathroom exhaust fan you create use a flow hood with a certain size hole. Instead I am going to use the building as the “flow hood.” I am going to measure the pressure differential between the outside and inside with no fan running. Then I am going to turn the fan on and measure the pressure difference. If I subtract those two outcome it will give me Pa. of the exhaust hood. Then, based off of a trusty YouTube video, the formula to convert the Pa. to CFM is 1.06 x sqinhole x squareroot of deltaPa will give me flow in CFM. I am trying to find this without having to set up a blower door. Any thoughts would be appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    I would welcome a comment by an experienced energy rater on this question.

  2. davidmeiland | | #2

    I don't have a good answer to this question, but I bet you could email the folks at Energy Conservatory and get some input.

    With a flow hood, the size of the hole is known, so a flow calculation is easy. In your restaurant example, the size of the hole really isn't known. Sure, you could close all the doors and windows, then open one door or window and measure the size of the hole... then use that hole size in your calculation... but you won't know if there are other large openings contributing to the hole size. You would need a blower door to estimate that.

    You might look into the "add a hole" method used in estimating the size of leakage openings in blower door testing,

    In any case, without a duct traverse, I think the best you can get is a very rough estimate. Please let us know what you find out.

  3. user-1105741 | | #3

    Hi Matt,

    Yes, this approach does work when you have a blower door to test the enclosure leakage, and seems to return reasonable numbers for us. We use it on every home we test to approximate the dryer and range hood flows. While I haven't compared actual flows using a more accurate device, we have compared before and after numbers where the flows of the device weren't modified, but the enclosure leakage was reduced by 60%, and the numbers were incredibly close and repeatably similar.

    The formula you will want to use is different than the one you found online.

    It is: (X/50)^0.65*Y where:
    X is the change in Pascals on your manometer between "on" and "off" fan when referencing inside and outside pressures
    Y is the CFM50 depressurization leakage test result on the enclosure.
    ^ is an exponent
    (tip: copy/paste this formula =(b1/50)^0.65*c1 in cell A1 of excel, and enter X value in b1 and Y value in c1).

    You'll want to do this on a very calm day, as the numbers start to move substantially with even light winds. The baseline feature of the manometer is handy, as is the time average.

    Use baseline for 30-60 seconds to zero the home, then "LONG" time average to get a 30-60 second average with the fan running, then shut it off and repeat again. Repeat a couple of times and average the results.

    The exhaust depressurization chart that follows this same formula is in your Energy Conservatory Blower Door manual on page 51. Online copy:

    Have fun.

  4. user-1105741 | | #4

    Sorry Matt- missed the ending part about not wanting to set up a blower door.

  5. user-1129453 | | #5

    Thanks for the responses David and Mike. That is Ok mike. I will keep info for future reference. The only reason I cant set up the blower door is because the restaurant is open from 6am until 8pm everyday with only one entry door.
    I wanted to give you all an update. After looking at the site, I have decided that it is going to be much less time consuming and more accurate to actually test the outside exhaust and supply vents on the roof of the building. That way I can make a box that has a specific hole size that will match up to the calculation and I wont have to guess at building leakage for the calculation. I am doing this out of the curiosity of the HVAC contractor. The inspector told him that he needs a certain amount of fresh air to come into the building through the return. (20% I think, which sounds high) The difference in the hood results (Exhaust/input) may make this hole in the return irrelevant only adding to the load. What do you all think about the fresh air rule without factoring in building leakage and hood exhaust/supply differential? The building is already very negative. You can tell that just by trying to open the doors. I understand fresh air and negative pressure is important but 20% fresh air in return sounds like a waste of energy. The only thing that I can think of is thinking about the two systems independently of each other would make the 20% rule would make sense but knowing that the systems are effecting the same room seems like the 20% rule is a bit excessive. Im just thinking out loud though. Thoughts or corrections? Set me straight if im missing something. Thanks!

  6. GBA Editor
    Martin Holladay | | #6

    This comment is off-topic, I know: but I can't help mentioning that it is illegal to invite the public to dine in a restaurant with a single entry door. That is a fire hazard and violates all building codes.

  7. BobHr | | #7

    Is there a makeup air system that runs when the exhaust hood runs. Without the makeup air the exhaust hood puts the building under a negative pressure. A restaurant manager I know has told me it is very hard to open an exterior door when the make up air is not working.

    Is there a point to measuring the hood flow if make up air is provided.

  8. user-917856 | | #8

    Similar problems exist in measuring air flow of residential hoods too. Some over-the-oven microwaves have built in exhaust ports, but the bottom of the appliance is not a flat plane. In which case, you can't place the Energy Conservatory flow pan. However, you can build a cardboard box around the hood, tape it for a good seal, and then place a manometer in it, and use the following webpage to calculate the flow:

    How to build the box:

    Anyone tried this method?

  9. davidmeiland | | #9

    There is an excellent discussion of flow measurement on the LinkedIn BPI/RESNET group right now, for anyone who has access.

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